<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      @media screen and (min-width: 320px) {
        html {
          font-size: 21.33px;
        }
      }
      @media screen and (min-width: 750px) {
        html {
          font-size: 50px;
        }
      }
      div {
        width: 2rem;
        height: 2rem;
        background-color: pink;
      }
    </style>
  </head>
  <body>
    <!-- 在 >=320px 时 div 宽度为 42.66px  -->
    <!-- 在 >=750px 时 div 宽度为    50px  -->
    <div></div>
  </body>
</html>
